1. Field of the Invention
The invention relates generally to the field of measurement while drilling systems. More specifically, the invention relates to devices for measuring parameters related to the shape of the interior wall of the wellbore, more commonly called “calipers.”
2. Background Art
Measurement while drilling (“MWD”) systems and methods generally include sensors disposed in or on components that are configured to be coupled into a “drill string.” A drill string is a pipe or conduit that is used to rotate a drill bit for drilling through subsurface rock formations to create a wellbore therethrough. A typical drill string is assembled by threadedly coupling end to end a plurality of individual segments (“joints”) of drill pipe. The drill string is suspended at the Earth's surface by a hoisting unit known as a “drilling rig.” The rig typically includes equipment that can rotate the drill string, or the drill string may include therein a motor that is operated by the flow of drilling fluid (“drilling mud”) through an interior passage in the drill string. During drilling a wellbore, some of the axial load of the drill string to the drill bit located at the bottom of the drill string. The equipment to rotate the drill string is operated and the combined action of axial force and rotation causes the drill bit to drill through the subsurface rock formations.
The drilling mud is pumped through the interior of the drill string by various types of pumps disposed on or proximate the drilling rig. The mud exits the drill string through nozzles or courses on the bit, and performs several functions in the process. One is to cool and lubricate the drill bit. Another is to provide hydrostatic pressure to prevent fluid disposed in the pore spaces of porous rock formations from entering the wellbore, and to maintain the mechanical integrity of the wellbore. The mud also lifts the drill cuttings created by the bit to the surface for treatment and disposal.
In addition to the above mentioned sensors, the typical MWD system includes a data processor for converting signals from the sensors into a telemetry format for transmission of selected ones of the signals to the surface. In the present context, it is known in the art to distinguish the types of sensors used in a drill string between those used to make measurements related to the geodetic trajectory of the wellbore and certain drilling mechanical parameters as “measurement while drilling” sensors, while other sensors, used to make measurements of one or more petrophysical parameters of the rock formations surrounding the wellbore are frequently referred to as “logging while drilling” (“LWD”) sensors. For purposes of the description of the present invention, the term MWD or “measurement while drilling” is intended to include both of the foregoing general classifications of sensors and systems including the foregoing, and it is expressly within the scope of the present invention to communicate any measurement whatsoever from a component in drill string to the surface using the method to be described and claimed herein below.
Communicating measurements made by one or more sensors in the MWD system is typically performed by the above mentioned data processor converting selected signals into a telemetry format that is applied to a valve or valve assembly disposed within a drill string component such that operation of the valve modulates the flow of drilling mud through the drill string. Modulation of the flow of drilling mud creates pressure variations in the drilling mud that are detectable at the Earth's surface using a pressure sensor (transducer) arranged to measure pressure of the drilling mud as it is pumped into the drill string. Forms of mud flow modulation known in the art include “negative pulse” in which operation of the valve momentarily bypasses mud flow from the interior of the drill string to the annular space between the wellbore and the drill string; “positive pulse” in which operation of the valve momentarily reduces the cross-sectional area of the valve so as to increase the mud pressure, and “mud siren”, in which a rotary valve creates standing pressure waves in the drilling mud that may be converted to digital bits by appropriate phasing of the standing waves. It is also known in the art to communicate signals between the surface and instrumentation in a wellbore using “wired” drill pipe,”, that is, segmented pipe having an electromagnetic communication channel associated therewith. See, e.g., U.S. Pat. No. 6,641,434 issued to Boyle et al. and assigned to the assignee of the present invention. It is also known in the art to use extremely low frequency (ELF) electromagnetic signal telemetry for such wellbore to surface signal communication.
It is frequently desirable to have information concerning the shape of the wellbore wall, for example, for calculating cement volume necessary to cement a pipe of casing in the wellbore. It is also desirable to know the distance between certain types of sensors and the wall of the wellbore, for example, acoustic, neutron and density sensors. Caliper devices known in the art for use in drill strings include acoustic travel time based devices. An acoustic transducer emits an ultrasonic pulse into the drilling fluid in the wellbore, and a travel time to the wellbore wall back to the transducer of the acoustic pulse is used to infer the distance from the transducer to the wellbore wall. There are circumstances in which such calipers are undesirable or fail to function properly, e.g., drilling fluid having entrained gas. It is also necessary to accurately determine the acoustic velocity of the drilling fluid proximate the caliper. Therefore, there exists a need for other types of wellbore calipers that can be used with drill strings.